pragma solidity ^0.4.16;

/*
 * Abstract Token Smart Contract.  Copyright © 2017 by ABDK Consulting.
 * Author: Mikhail Vladimirov <mikhail.vladimirov@gmail.com>
 */
pragma solidity ^0.4.20;

/*
 * EIP-20 Standard Token Smart Contract Interface.
 * Copyright © 2016–2018 by ABDK Consulting.
 * Author: Mikhail Vladimirov <mikhail.vladimirov@gmail.com>
 */
pragma solidity ^0.4.20;

/**
 * ERC-20 standard token interface, as defined
 * <a href="https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md">here</a>.
 */
contract Token {
  /**
   * Get total number of tokens in circulation.
   *
   * @return total number of tokens in circulation
   */
  function totalSupply () public view returns (uint256 supply);

  /**
   * Get number of tokens currently belonging to given owner.
   *
   * @param _owner address to get number of tokens currently belonging to the
   *        owner of
   * @return number of tokens currently belonging to the owner of given address
   */
  function balanceOf (address _owner) public view returns (uint256 balance);

  /**
   * Transfer given number of tokens from message sender to given recipient.
   *
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer to the owner of given address
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transfer (address _to, uint256 _value)
  public returns (bool success);

  /**
   * Transfer given number of tokens from given owner to given recipient.
   *
   * @param _from address to transfer tokens from the owner of
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer from given owner to given
   *        recipient
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transferFrom (address _from, address _to, uint256 _value)
  public returns (bool success);

  /**
   * Allow given spender to transfer given number of tokens from message sender.
   *
   * @param _spender address to allow the owner of to transfer tokens from
   *        message sender
   * @param _value number of tokens to allow to transfer
   * @return true if token transfer was successfully approved, false otherwise
   */
  function approve (address _spender, uint256 _value)
  public returns (bool success);

  /**
   * Tell how many tokens given spender is currently allowed to transfer from
   * given owner.
   *
   * @param _owner address to get number of tokens allowed to be transferred
   *        from the owner of
   * @param _spender address to get number of tokens allowed to be transferred
   *        by the owner of
   * @return number of tokens given spender is currently allowed to transfer
   *         from given owner
   */
  function allowance (address _owner, address _spender)
  public view returns (uint256 remaining);

  /**
   * Logged when tokens were transferred from one owner to another.
   *
   * @param _from address of the owner, tokens were transferred from
   * @param _to address of the owner, tokens were transferred to
   * @param _value number of tokens transferred
   */
  event Transfer (address indexed _from, address indexed _to, uint256 _value);

  /**
   * Logged when owner approved his tokens to be transferred by some spender.
   *
   * @param _owner owner who approved his tokens to be transferred
   * @param _spender spender who were allowed to transfer the tokens belonging
   *        to the owner
   * @param _value number of tokens belonging to the owner, approved to be
   *        transferred by the spender
   */
  event Approval (
    address indexed _owner, address indexed _spender, uint256 _value);
}
/*
 * Safe Math Smart Contract.  Copyright © 2016–2017 by ABDK Consulting.
 * Author: Mikhail Vladimirov <mikhail.vladimirov@gmail.com>
 */
pragma solidity ^0.4.20;

/**
 * Provides methods to safely add, subtract and multiply uint256 numbers.
 */
contract SafeMath {
  uint256 constant private MAX_UINT256 =
    0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF;

  /**
   * Add two uint256 values, throw in case of overflow.
   *
   * @param x first value to add
   * @param y second value to add
   * @return x + y
   */
  function safeAdd (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    assert (x <= MAX_UINT256 - y);
    return x + y;
  }

  /**
   * Subtract one uint256 value from another, throw in case of underflow.
   *
   * @param x value to subtract from
   * @param y value to subtract
   * @return x - y
   */
  function safeSub (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    assert (x >= y);
    return x - y;
  }

  /**
   * Multiply two uint256 values, throw in case of overflow.
   *
   * @param x first value to multiply
   * @param y second value to multiply
   * @return x * y
   */
  function safeMul (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    if (y == 0) return 0; // Prevent division by zero at the next line
    assert (x <= MAX_UINT256 / y);
    return x * y;
  }
}


/**
 * Abstract Token Smart Contract that could be used as a base contract for
 * ERC-20 token contracts.
 */
contract AbstractToken is Token, SafeMath {
  /**
   * Create new Abstract Token contract.
   */
  function AbstractToken () public {
    // Do nothing
  }

  /**
   * Get number of tokens currently belonging to given owner.
   *
   * @param _owner address to get number of tokens currently belonging to the
   *        owner of
   * @return number of tokens currently belonging to the owner of given address
   */
  function balanceOf (address _owner) public view returns (uint256 balance) {
    return accounts [_owner];
  }

  /**
   * Transfer given number of tokens from message sender to given recipient.
   *
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer to the owner of given address
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transfer (address _to, uint256 _value)
  public returns (bool success) {
    uint256 fromBalance = accounts [msg.sender];
    if (fromBalance < _value) return false;
    if (_value > 0 && msg.sender != _to) {
      accounts [msg.sender] = safeSub (fromBalance, _value);
      accounts [_to] = safeAdd (accounts [_to], _value);
    }
    Transfer (msg.sender, _to, _value);
    return true;
  }

  /**
   * Transfer given number of tokens from given owner to given recipient.
   *
   * @param _from address to transfer tokens from the owner of
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer from given owner to given
   *        recipient
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transferFrom (address _from, address _to, uint256 _value)
  public returns (bool success) {
    uint256 spenderAllowance = allowances [_from][msg.sender];
    if (spenderAllowance < _value) return false;
    uint256 fromBalance = accounts [_from];
    if (fromBalance < _value) return false;

    allowances [_from][msg.sender] =
      safeSub (spenderAllowance, _value);

    if (_value > 0 && _from != _to) {
      accounts [_from] = safeSub (fromBalance, _value);
      accounts [_to] = safeAdd (accounts [_to], _value);
    }
    Transfer (_from, _to, _value);
    return true;
  }

  /**
   * Allow given spender to transfer given number of tokens from message sender.
   *
   * @param _spender address to allow the owner of to transfer tokens from
   *        message sender
   * @param _value number of tokens to allow to transfer
   * @return true if token transfer was successfully approved, false otherwise
   */
  function approve (address _spender, uint256 _value)
  public returns (bool success) {
    allowances [msg.sender][_spender] = _value;
    Approval (msg.sender, _spender, _value);

    return true;
  }

  /**
   * Tell how many tokens given spender is currently allowed to transfer from
   * given owner.
   *
   * @param _owner address to get number of tokens allowed to be transferred
   *        from the owner of
   * @param _spender address to get number of tokens allowed to be transferred
   *        by the owner of
   * @return number of tokens given spender is currently allowed to transfer
   *         from given owner
   */
  function allowance (address _owner, address _spender)
  public view returns (uint256 remaining) {
    return allowances [_owner][_spender];
  }

  /**
   * Mapping from addresses of token holders to the numbers of tokens belonging
   * to these token holders.
   */
  mapping (address => uint256) internal accounts;

  /**
   * Mapping from addresses of token holders to the mapping of addresses of
   * spenders to the allowances set by these token holders to these spenders.
   */
  mapping (address => mapping (address => uint256)) internal allowances;
}


/**
 * Native Video Box token smart contract.
 */
contract NVBToken is AbstractToken {
  /**
   * Address of the owner of this smart contract.
   */
  address private owner;

  /**
   * Total number of tokens in circulation.
   */
  uint256 tokenCount;

  /**
   * True if tokens transfers are currently frozen, false otherwise.
   */
  bool frozen = false;

  /**
   * Create new Native Video Box token smart contract, with given number of tokens issued
   * and given to msg.sender, and make msg.sender the owner of this smart
   * contract.
   *
   * @param _tokenCount number of tokens to issue and give to msg.sender
   */
  function NVBToken (uint256 _tokenCount) public {
    owner = msg.sender;
    tokenCount = _tokenCount;
    accounts [msg.sender] = _tokenCount;
  }

  /**
   * Get total number of tokens in circulation.
   *
   * @return total number of tokens in circulation
   */
  function totalSupply () public view returns (uint256 supply) {
    return tokenCount;
  }

  /**
   * Get name of this token.
   *
   * @return name of this token
   */
  function name () public pure returns (string result) {
    return "NVB";
  }

  /**
   * Get symbol of this token.
   *
   * @return symbol of this token
   */
  function symbol () public pure returns (string result) {
    return "NVB";
  }

  /**
   * Get number of decimals for this token.
   *
   * @return number of decimals for this token
   */
  function decimals () public pure returns (uint8 result) {
    return 10;
  }

  /**
   * Transfer given number of tokens from message sender to given recipient.
   *
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer to the owner of given address
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transfer (address _to, uint256 _value)
    public returns (bool success) {
    if (frozen) return false;
    else return AbstractToken.transfer (_to, _value);
  }

  /**
   * Transfer given number of tokens from given owner to given recipient.
   *
   * @param _from address to transfer tokens from the owner of
   * @param _to address to transfer tokens to the owner of
   * @param _value number of tokens to transfer from given owner to given
   *        recipient
   * @return true if tokens were transferred successfully, false otherwise
   */
  function transferFrom (address _from, address _to, uint256 _value)
    public returns (bool success) {
    if (frozen) return false;
    else return AbstractToken.transferFrom (_from, _to, _value);
  }

  /**
   * Change how many tokens given spender is allowed to transfer from message
   * spender.  In order to prevent double spending of allowance, this method
   * receives assumed current allowance value as an argument.  If actual
   * allowance differs from an assumed one, this method just returns false.
   *
   * @param _spender address to allow the owner of to transfer tokens from
   *        message sender
   * @param _currentValue assumed number of tokens currently allowed to be
   *        transferred
   * @param _newValue number of tokens to allow to transfer
   * @return true if token transfer was successfully approved, false otherwise
   */
  function approve (address _spender, uint256 _currentValue, uint256 _newValue)
    public returns (bool success) {
    if (allowance (msg.sender, _spender) == _currentValue)
      return approve (_spender, _newValue);
    else return false;
  }

  /**
   * Burn given number of tokens belonging to message sender.
   *
   * @param _value number of tokens to burn
   * @return true on success, false on error
   */
  function burnTokens (uint256 _value) public returns (bool success) {
    if (_value > accounts [msg.sender]) return false;
    else if (_value > 0) {
      accounts [msg.sender] = safeSub (accounts [msg.sender], _value);
      tokenCount = safeSub (tokenCount, _value);

      Transfer (msg.sender, address (0), _value);
      return true;
    } else return true;
  }

  /**
   * Set new owner for the smart contract.
   * May only be called by smart contract owner.
   *
   * @param _newOwner address of new owner of the smart contract
   */
  function setOwner (address _newOwner) public {
    require (msg.sender == owner);

    owner = _newOwner;
  }

  /**
   * Freeze token transfers.
   * May only be called by smart contract owner.
   */
  function freezeTransfers () public {
    require (msg.sender == owner);

    if (!frozen) {
      frozen = true;
      Freeze ();
    }
  }

  /**
   * Unfreeze token transfers.
   * May only be called by smart contract owner.
   */
  function unfreezeTransfers () public {
    require (msg.sender == owner);

    if (frozen) {
      frozen = false;
      Unfreeze ();
    }
  }

  /**
   * Logged when token transfers were frozen.
   */
  event Freeze ();

  /**
   * Logged when token transfers were unfrozen.
   */
  event Unfreeze ();
}